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CE4000 Cable Explorer TDR
User Manual
30
VOP
The velocity of propagation is the speed at which the signal
travels on the cable under test. The time required for the
television signals to reach the customers is normally not a
concern, however some delay is present in the system, due
primarily to the delay of the cable and filters in the network.
The VOP of a cable is the speed at which the signal travels
divided by the speed of light and is expressed as a decimal or as
a percentage (i.e. .85 or 85%). Most hard line cable will have a
VOP of .87 or 87% or better. Typical (newer) drop cable will
have a VOP of 80% to 85%. Older drop cables may have VOPs
as low as 70%.
Approximately 50 cables are pre-programmed into the CE4000
for use. The user may add, delete or edit this data table. (VOP
specifications are available from the cable manufacturer’s data
sheet.) As shown in the previous equation, TDR simply measures
the time between the incidence and reflected pulse and calculates
the distance to the fault. Obviously, the accuracy of the VOP has
a direct relationship to the accuracy of the resultant distance
measurements. A 1% error in the VOP will result in a 1% error
in the distance measurement, plus any inaccuracy in the time
measurement capability of the TDR. A cable’s VOP can also be
measured if the cable’s length is known by using the same
formula. This procedure is detailed in the Operation section of
this manual.
PULSE WIDTH
The pulse is 1/2 of a sine wave (negative pulse, but displayed as
positive). The pulse width is typically programmable, so that
longer pulses can be used for longer ranges—pulses with more
energy can travel longer distances. The cable under test exhibits
the same loss characteristics on the incidence pulse as they do on
the typical RF signals on the network. The longer the cable is, the
higher the loss. Note that the reflected pulse has traveled down
the cable and back, thus it has experienced twice the cable
attenuation.